Comey came under fire in the last days of the presidential campaign when he sent a letter to Congress informing it of further investigations into Clinton's private email server.

That announcement was made just 10 days before the election took place – an event that many, including Clinton, claimed contributed to her defeat.

In Trump's dismissal letter to Comey, Trump wrote: "While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the Department of Justice that you are not able to effectively lead the bureau."

Senate Democratic Leader Chuck Schumer said he told President Trump he was making a "big mistake" in firing Comey. The senator said that the investigation into Russia's meddling in the 2016 election should be "as far away as possible" from the White House.

Congressman Elijah Cummings, ranking member of the house committee on oversight and government reform, has called for "immediate emergency hearings" on Comey's termination.

"Congress needs to have immediate emergency hearings to obtain testimony directly from Attorney General Sessions, the deputy attorney general, and FBI Director Comey," Cummings said in a statement.

"The White House was already covering up for Michael Flynn by refusing to provide a single document to Congress, and now the President fired the one independent person who was doing the most to investigate President Trump and his campaign over allegations of coordination with Russia.

"It is mindboggling that the Attorney General – who claimed to have recused himself – was directly involved in the decision to fire Director Comey according to the White House itself. There is now a crisis of confidence at the Justice Department, and President Trump is not being held accountable because House Republicans refuse to work with us to do our job.

"Congress must restore credibility, accountability, and transparency to this investigation and finally pass legislation to create a truly independent commission," said Cummings.

'You are making a big a mistake' Chuck Schumer tells Trump over firing FBI director James ComeyReuters